1. Technical Field
The present invention relates to a battery capacity controller that corrects a battery capacity or a storage capacity of a battery obtained by a calculation.
2. Related Art
On a vehicle such as an EV (Electric Vehicle) or an HEV (Hybrid Electrical Vehicle), a battery is mounted that supplies an electric power to an electric motor. In the battery, a plurality of battery cells connected in series are provided. As the battery cell, a secondary cell such as a nickel hydrogen cell or a lithium ion is used. In this case, in order to use the secondary cell safely, a residual capacity (SOC:State of Charge) of each battery cell needs to be constantly monitored to prevent an overcharge or an over-discharge.
As one of methods for detecting the SOC of the battery cell, a method has been known in which the SOC is calculated on the basis of the integration of a current. In this method, the charging current and the discharging current of the battery cell are integrated at intervals of prescribed periods to calculate a quantity of integrated charging current and a quantity of integrated discharging current and the quantity of integrated charging current and the quantity of integrated discharging current is added to or subtracted from the SOC of an initial state or the SOC immediately before the start of charging and discharging operations to obtain the SOC of the battery cell. The SOC calculated by this method is referred to as an “integrated SOC”, hereinafter.
An accuracy of the integrated SOC obtained by the above-described method is high. However, since, in the battery mounted on the vehicle, charging and discharging operations are repeated within a prescribed width of the SOC of the battery cell and the battery cell is used for a long period, errors are accumulated in the integrated SOC. Further, when the quantity of integrated charging current the quantity of integrated discharging current are calculated, for instance, measurement errors of a current detector may be occasionally accumulated to increase the error of the integrated SOC. Further, since the decrease of the capacity due to a self-discharge occurring while the vehicle is left for a long period is not integrated, this also results in a factor of the error.
As another method for detecting the SOC of the battery cell, there is a method for detecting the SOC on the basis of a terminal voltage of the battery cell during the charging and discharging operations. A prescribed relation exists between the SOC of the battery cell and an open circuit voltage (OCV) and one example thereof is shown in FIG. 11. An upper limit SOC and a lower limit SOC in FIG. 11 show an upper end value and a lower end value within a prescribed range of the SOC where the charging and discharging operations of the battery cell are repeated. In the battery cell having a property shown in FIG. 11, the change of the OCV is large in the upper limit SOC and the lower limit SOC, however, in a range between the upper limit SOC and the lower limit SOC, the change of the OCV is very small.
Accordingly, when the SOC of the battery cell having the property shown in FIG. 11 is calculated in accordance with the terminal voltage, the upper limit SOC and the lower limit SOC can be accurately obtained, however, the SOC ranging from the upper limit SOC to the lower limit SOC cannot be accurately obtained. Further, as described above, since in the battery mounted on the vehicle, the charging and discharging operations are repeated within the prescribed width of the SOC of the battery cell, the influence of a diffusion resistance is large and the OCV property of the battery cell is greatly changed. As described above, the accuracy of the SOC calculated on the basis f the terminal voltage is frequently low.
The above-described two methods respectively have advantages and disadvantages. However, when the disadvantages are respectively compensated for each other, an accurate SOC is supposed to be obtained. A residual capacity detector disclosed in patent literature 1 replaces data of an integrated residual capacity by a prescribed upper limit value in accordance with the terminal voltage of a battery device and carries out a correcting process so that a difference between an integrated residual capacity calculated after the replacement of the data and a corrected residual capacity obtained by correcting the integrated residual capacity is increased in accordance with the increase of the integrated value of charging and discharging currents. Accordingly, for instance, even when a deviation between the integrated residual capacity and an actual residual capacity is increased in accordance with the increase of the integrated value of the charging and discharging currents, the accuracy of an approximation of the corrected residual capacity to the actual residual capacity can be improved.    [Patent literature 1] JP-A-2002-328154    [Patent literature 2] JP-A-2002-286820    [Patent literature 3] JP-A-2006-215001    [Patent literature 4] JP-A-11-346444    [Patent literature 5] Japanese Patent No. 3864590    [Patent literature 6] Japanese Patent No. 3752879    [Patent literature 7] Japanese Patent No. 3454657    [Patent literature 8] Japanese Patent No. 3767150
When a detecting accuracy of the SOC of the battery is not good, after the charging and discharging operations of the battery, a state may possibly occur that the SOC deviates from a range of the SOC where the battery can be used. For instance, even when the actual SOC is lower than the lower limit SOC, the above-described state may possibly occur. Such an undesirable state may possibly shorten the life of the battery or affect the control of the electric motor using the battery. The above-described possibility is low in the first stage that the use of the battery is started, however, is high in the last stage that the battery is used for a long time to come near to an end of its span of life.
Accordingly, particularly in the last stage of the battery, a good detecting accuracy of the SOC is required. As described above, the integrated SOC calculated on the basis of the integration of the current has a high accuracy, however, includes an error. Further, in the SOC calculated on the basis of the terminal voltage, since the change of the OCV is large in the vicinity of the upper limit SOC and the lower limit SOC, the accuracy is high. However, in the range between the upper limit SOC and the lower limit SOC, since the change of the OCV is small, the accuracy is low. Accordingly, a method is supposed to be used in which the SOC of the battery is ordinarily managed in accordance with the integrated SOC calculated on the basis of the integration of the current, however, when an actual SOC comes near to the upper limit SOC or the lower limit SOC, the SOC is corrected by an SOC calculated on the basis of the terminal voltage.
FIGS. 12A and 12B show the ranges of a control SOC and an actual SOC of the battery in the first stage (FIG. 12A) and the last stage (FIG. 12B). Further, FIGS. 13A and 13B show graphs illustrating a transition of the decreasing control SOC of the battery in the first stage (FIG. 13A) and the last stage (FIG. 13B). Further, FIG. 14 is a diagram showing the output change of the battery that is changed by the control of the decreasing control SOC in the battery of the last stage. The control SOC indicates an SOC of the battery that is recognized by an ECU for managing the state of the battery. In examples shown in FIGS. 12 and 13, the battery is used when the control SOC is located between 20% and 80%. Further, the actual SOC indicates an actual SOC of the battery at that time.
As shown in FIGS. 12A and 13A, in the case of the battery of the first stage, even when the ECU decides that the control SOC is lowered to, for instance, 28% due to a discharging operation for supplying an electric power to an electric motor, the actual SOC is not lowered to 28%. Therefore, although the discharging operation is continuously carried out, the ECU maintains the control SOC to 28% as it is, and then, when the SOC calculated on the basis of the terminal voltage reaches 20%, the ECU corrects the control SOC to 20%.
On the other hand, as shown in FIGS. 12B and 13B, the capacity of the battery of the last stage is decreased more than the capacity of the battery of the first stage. Accordingly, in the case of the battery of the last stage, when the ECU decides that the control SOC is lowered to, for instance, 53%, the actual SOC is lowered to 20%. Therefore, the ECU corrects the control SOC from 53% to 20% in accordance with the SOC (20%) calculated on the basis of the terminal voltage. As a result, as shown in FIG. 14, the output of the battery is greatly lowered.
Now, a case will be described in which the control SOC of the battery is increased. FIGS. 15A and 15B show the ranges of the control SOC and the actual SOC of the battery in the first stage (FIG. 15A) and the last stage (FIG. 15B). Further, FIGS. 16A and 16B show graphs illustrating a transition of the increasing control SOC of the battery in the first stage (FIG. 16A) and the last stage (FIG. 16B). Further, FIG. 17 is a diagram showing the output change of the battery that is changed by the control of the increasing control SOC in the battery of the last stage.
As shown in FIGS. 15A and 16A, in the case of the battery of the first stage, even when the ECU decides that the control SOC is increased to, for instance, 72% due to a charging operation of the battery, the actual SOC is not increased to 72%. Therefore, although the charging operation is continuously carried out, the ECU maintains the control SOC to 72% as it is, and then, when the SOC calculated on the basis of the terminal voltage reaches 80%, the ECU corrects the control SOC to 80%.
On the other hand, as shown in FIGS. 15B and 16B, the capacity of the battery of the last stage is decreased more than the capacity of the battery of the first stage. Accordingly, in the case of the battery of the last stage, when the ECU decides that the control SOC is increased to, for instance, 47%, the actual SOC is increased to 80%. Therefore, the ECU corrects the control SOC from 47% to 80% in accordance with the SOC (80%) calculated on the basis of the terminal voltage. As a result, as shown in FIG. 17, the output of the battery is greatly lowered.
An output torque of the electric motor as a driving source of the vehicle such as the EV or the HEV is controlled in accordance with the SOC of the battery, because an output is different depending on the SOC of the battery as shown in FIGS. 14 and 17. Accordingly, an impression of a driver about the traveling performance of the vehicle may possibly depend on an influence given to the electric motor in accordance with the SOC of the battery. For instance, during the cruising travel of the vehicle, the driver slightly steps on an accelerator and an assist force by the electric motor in the HEV is low. At this time, even when the control SOC is corrected by the SOC calculated on the basis of the terminal voltage, an influence given to the assist force by the electric motor is low.
However, at the time of acceleration or climbing a slope, the driver strongly steps on the accelerator and the assist force by the electric motor in the HEV is high. At this time, when the control SOC is corrected and, for instance, the control SOC is abruptly lowered as shown in FIG. 14, the electric power supplied to the electric motor from the battery is abruptly decreased to lower the output torque of the electric motor. Accordingly, although the driver stepson the accelerator, a desired torque is not outputted or the torque is abruptly changed. Thus, the driver has an uneasy feeling for the traveling performance.
When the battery continuously supplies the electric power to the electric motor not to give such an uneasy feeling to the driver, though the actual SOC of the battery is the lower limit SOC or lower, the battery needs to continuously output to the electric motor the electric power not lower than the performance of the battery. Such a control undesirably causes the life of the battery to be shortened or an adverse effect to be given to the durability of the battery.